1. Field of the Invention
The invention relates to a switched-mode power supply comprising input terminals for receiving an input voltage and output terminals for supplying a DC output voltage to a load; a capacitance and an inductance; charging means for receiving the input voltage, coupled to the capacitance for periodically storing a charge in the capacitance during a first period; rectifier means coupled between the inductance and one of the output terminals to supply the DC output voltage; a switching element coupled to the inductance; and a control circuit for controlling the switching element in an on or off-state, in which the switching element is periodically in the on-state during a second period. The invention also relates to a display apparatus comprising such a switched-mode power supply.
Such a switched-mode power supply is especially suitable for supplying a standby power in apparatuses in which parts of the circuits have to be active in a standby mode.
2. Description of the Related Art
In a known non-switched-mode power supply, a transformer is used which has a primary winding connected to the AC line. A secondary winding supplies a low voltage which, after stabilization, is suitable for supplying a low power to circuits in a television receiver which have to be active in the standby mode. Such a transformer, which is suitable for transforming the high line voltage to a low standby voltage at the low line frequency, is bulky. Such a non-switched-mode power supply has a very low efficiency due to stabilization losses which are extremely high if the line voltage varies over a large range. An additional main switched-mode power supply is available to supply a large power during normal operating conditions of the receiver. This main switched-mode power supply may be of any known type. All these main switched-mode supplies have in common that energy from the line is transferred to the load via a magnetic field in an inductor. To avoid bulky inductors, a high switching frequency of the main switched-mode power supply has to be selected.
It is also known that the main switched-mode power supply is used for generating the low standby power. Such a main switched-mode power supply, which is designed to supply a high output power, operates with a low efficiency if the low output power is supplied. For a large part, this is caused by the switching losses in the components and no-load losses in the system. At a low load, these no-load losses are relatively high. These switching losses depend heavily on the switching frequency. Therefore, solutions to improve the efficiency have been found by lowering the switching frequency of the main switched-mode power supply in the standby mode. However, the switching frequency has to be selected above the highest audible frequency to avoid irritation of the users by the noise generated by the switched-mode power supply. On the other hand, a very low switching frequency results in very large inductors and is not a viable alternative.
It is also known that a separate switched-mode power supply is used for supplying a small standby power in a television apparatus. Such a separate switched-mode power supply can be optimized for supplying a low power at a high efficiency. However, for the same reasons as given above, the switching frequency has to be selected above the highest audible frequency to avoid audible noise or too large inductive components. Although these separate optimized switched-mode power supplies yield a better efficiency in supplying a low power than the main switched-mode power supplies, the efficiency can still be improved. The separate switched-mode power supply is often a flyback converter. Such a flyback converter comprises input terminals to receive an AC line voltage. A series arrangement of a rectifier circuit and a smoothing capacitor is arranged across the input terminals to receive the AC line voltage. A series arrangement of an inductor and a main current path of a transistor is connected across the smoothing capacitor. Energy is stored in the inductor during an on-time of the transistor. The energy in the inductor is transferred to a load via a rectifier diode during the off-time of the transistor. A control circuit controls the switching transistor to switch at a frequency which is selected high enough to avoid audible noise. In most cases, dissipative snubber circuits are arranged in parallel with the inductor to avoid too steep variations of voltages or too high voltages in the circuit. The high switching frequency and the dissipative snubber cause the known separate switched-mode power supplies to have a non-optimal efficiency.